Classical Structural Theory

Among the problems confronting organic chemists, two especially seemed to test the limits of the classical structure theories, namely, benzene and acetoacetic ester, also known as ethyl acetoacetate. Arthur Lachman, a Munich-... 

Classical structure theory was developed purely from chemical facts, without any help from physics. The theory of resonance was well on its way toward formulation before quantum mechanics was discovered.. . . ... 

It is true that the idea of resonance energy was then provided by quantum mechanics. . . but the theory of resonance in chemistry has gone far beyond the region of application in which any precise quantum mechanical calculations have been made, and its great extension has been almost entirely empirical.. . . The theory of resonance in chemistry is an essentially qualitative theory, which, like the classical structure theory, depends for its successful application largely upon a chemical feeling that is developed through practice. 46... 

Pure substances. Language The Classical Structural Theory... 

In fact, as we will see, the classical structural theory provides the only means by which a chemist can visualise a synthesis. All science needs a language and the language of organic chemistry is the Classical Structural Theory. That this is so can be seen if the first attempts to synthesise quinine are considered. 

In Chapter 1 we have stated that the classical structural theory is the only way to "visualise" the synthesis of a more or less complex organic compound. However, all or most of the information given by a structural formula can also be expressed.by a matrix (see also Appendix A-1). There are different kinds of matrices for example, the adjacency matrix J, which originates in graph theory and indicates only which atoms are bonded, or the connectivity matrix C, whose off-diagonal entries are the formal covalent bond orders. For instance, the corresponding matrices of hydrogen cyanide are ... 

The essential identity in character of the theory of resonance and the classical structure theory of organic chemistry, which has before been referred to only briefly,42 will be discussed in detail in the following paragraphs. 

I feel that the greatest advantage of the theory of resonance, as compared with other ways (such as the molecular-orbital method) of discussing the structure of molecules for which a single valence-bond structure is not enough, is that it makes use of structural elements with which the chemist is familiar. The theory should not be assessed as inadequate because of its occasional unskillful application. 11 becomes more and more powerful, just as does classical structure theory, as the chemist develops a better and better chemical intuition about it. 

Fig. 11 is a drawing of a two-dimensional analogue of the electron-domain model of ethane. Large circles represent valence-shell electron-domains (superimposed on them are the valence strokes of classical structural theory). Plus signs represent protons of the "C—H bonds. The nuclei of the two carbon atoms are represented by small dots in the trigonal interstices of the electron-pair lattice. While these nuclei would not necessarily be in the centers of their interstices, exactly, it can be asserted that an (alchemical) insertion of the two protons on the... 

Multicenter Bonding. While the valence stroke of classical structural theory is for many purposes a useful representation of an electron-domain created by the fields of two atomic cores (for every line-segment... 

Linnett s procedure may be viewed as a refinement of classical structural theory In Linnett s theory, the van t Hoff-Lewis tetrahedral model is applied twice, once to each set of spins, with the assumption that, owing to coulombic repulsions between electrons of opposite spin, there may be, in some instances, a relatively large degree of spatial anticoincidence between a system s two spin-sets. 

The Exclusion Principle endows quantum mechanical systems with a property analogous in many respects to the classical concept of impenetrability 156h This property finds expression in classical structural theory in the concept of molecular, van der Waals domains that may touch and deform one another but do not overlap in the concept of ionic spheres of influence that, while polarizable and compressible, are effectively impenetrable and in the well known, if seldom articulated, theorem that the valence strokes of classical structural theory never cross one another 78h Taken with Lewis s identification of the valence-stroke as precisely two electrons, this non-crossing theorem virtually demands (in retrospect) a wave-like character for electrons and an exclusion principle. 

Nevertheless, acknowledging or denying the existence of differences between resonance theory and classical structural theory was dependent on their different assessments of the role of alternative methods to study molecular structure. Wheland equated resonance theory to the valence bond method and viewed them as alternatives to the molecular orbital method. Pauling conceded that the valence bond method could be compared with the molecular orbital method, but not with... 

In discussing resonance, Pauling stated "The theory of resonance in chemistry is essentially a qualitative theory, which like the classical structural theory, depends for its successful application largely upon a chemical feeling that is developed through practice."46 The same is true with aromaticity. 

In the framework of classical structure theory, a reaction can be described as a mechanistic reaction if there exists only one element -Dnux — of the dynamic sublattice that can be labeled by... 

Two points should be emphasized. First, according to classical structure theory, all the equivalent positions of a given set should be occupied and moreover they should all be occupied by atoms of the same kind. In later chapters we shall note examples of crystals in which one or both of these criteria are not satisfied an obvious case is a solid solution in which atoms of different elements occupy at random one or more sets of equivalent positions. (The occupation of different sets of equivalent positions by atoms of the same kind occurs frequently and may lead to quite different environments of chemically similar atoms. Examples include the numerous crystals in which there is both tetrahedral and octahedral coordination of atoms of the same element—in the same oxidation state—as noted in Chapter 5, and crystals in which there is both coplanar and tetrahedral coordination of Cu(ii), p. 890, or Ni(ii), p. 965.) The second point for emphasis is if a molecule (or complex ion) is situated at one of the special positions it should possess the point symmetry of that position. A molecule lying on a plane of symmetry must itself possess a plane of symmetry, and one having its centre at the intersection of two planes of symmetry must itself possess two perpendicular planes of symmetry. If, therefore, it can be demonstrated that a molecule lies at such a position as, for example, would be the case if the unit cell of Fig. 2.13 contained only one molecule, (a fact deducible from the density of the crystal) this would constitute a proof of the symmetry of the molecule. Such a conclusion is not, of course, valid if there is any question of random orientation or free rotation of the molecules. Moreover, there is another reason for caution in applying this type of argument to inorganic crystals. 

Pauling, L. In Perpectives in Organic Chemistry, pp. 1-8 (A. Todd, Ed.), New York Interscience, 1956. In this paper Pauling proffered the opinion that the resonance theory is an extension of the classical structural theory, rather than quantum-mechanical in character. 

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